This project will investigate several classical cellular communication mechanisms and how these are integrated with one another to produce a coordinated functioning characteristic of multicellular systems. The salivary gland cells of pulmonate molluscs provide a unique opportunity for such a study because they display a broad spectrum of cellular communication processes in a highly tractable preparation. These extremely large cells display extensive intercellular communication with one another via low resistance junctions. Furthermore, these cells, though previously thought of as "non-excitable," generate all-or-none, overshooting action potentials essentially indistinguishable from those generated by neurons. Another important feature is that these giant cells allow extremely high resolution of the cellular events underlying secretion. Both the secretory process and the communication between cells by electrical coupling may be modulated by intracellular calcium concentrations. This possibility will be given prominence in these studies because calcium appears to carry the major inward current for the generation of action potentials. These investigations will characterize the ionic and metabolic bases of resting and action potentials. This will produce a model for such phenomena in other classically "non-excitable" systems where such experiments are technically too difficult. Thus, these studies provide a unique opportunity to examine the coordinated interaction of events, as calcium-mediated action potentials propagate from cell to cell via low resistance pathways and apparently provide the key event in initiating stimulus-secretion coupling in this exocrine system.